Open FIR, therefore you see and avoid. Don't rely on anything, especially not an FIS.
There is no requirement for the other aircraft to be talking to anyone and it is best not to assume that if you are talking to one station, they won't be talking to another, if any.
Keep your eyes open and as eharding has mentioned, don't get all aggressive about it. That is simply how it is. They wouldn't have descended in front of you had they seen you, that is for sure.
You made the mistake of assuming traffic would be aware of you and maybe the other a/c made the mistake of not talking to the same unit as you. It doesn't matter who was in the wrong if you are tumbling out of the sky after a mid air.
You've already had good advice on this thread. The best advice would be to absorb it. FIS and RIS give absolutely no guarantees of traffic avoidance and only a fool would rely on them, especially if flying VFR in good conditions.
It is a big sky, but you need to know the limitations of your a/c, especially in regard to blindspots and the problems associated with high wing and low wing machines whilst looking out.
Take it as a learning experience and don't get all aggresive about mistake you percieve others to have made. It just makes you sound like a nobber.